Communication devices such as mobile stations are also known as e.g. mobile terminals, wireless terminals and/or User Equipments (UEs). Mobile stations are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system. The communication may be performed e.g. between two mobile stations, between a mobile station and a regular telephone and/or between a mobile station and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Mobile stations may further be referred to as mobile telephones, cellular telephones, or laptops with wireless capability, just to mention some further examples. The mobile stations in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another mobile station or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the mobile stations within range of the base stations.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
In the context of this disclosure, the expression DownLink (DL) is used for the transmission path from the base station to the mobile station. The expression UpLink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
At traffic congestion in the radio access network the resources of a base station such as an eNB or a cell may be depleted such that no more bearers for end users such as user equipments can be set up in the base station or the cell. A user equipment wishing to access a service via the radio access network will experience a denial of the requested service. For high priority services, like emergency calls, some of the “soft” resource limits set up by the operator of the radio access network may be disregarded to still carry out the high priority service. Examples of such “soft” limits are e.g. a licensed number of connected user equipments, and a bandwidth allocated for GBR bearers. A GPR bearer is a bearer with guaranteed bit rate. A user equipment may have many simultaneous bearers, each bearer is associated with a certain Quality of Service. Services with the same quality of service share one bearer
The licensed number of connected user equipments is normally not a critical limit from a physical resource point of view and can be overridden without any immediate consequences.
When a GBR bearer is allocated for a user equipment, a certain bandwidth is reserved for this bearer for the lifetime of the bearer. The total bandwidth set aside for GBR bearers represents a physical resource but this reservation can be temporarily extended to allow set up of high priority GBR bearers. Some bandwidth must however be reserved for non GBR bearers to avoid total “starvation” of such bearers if all allocated GBR bandwidth should be fully utilized at some point in time. Typically not all user equipments utilize their allocated bandwidth all the time so there will normally be plenty of bandwidth which can be used by the non-GBR bearers. However, when the reservation for GBR bearers has been extended to its limit, the only way to still admit another GBR bearer is to enforce a release of an already established bearer. This may be performed by so called pre-emption, i.e. immediate release of some active GBR bearer to make room for the new one. Another way is to apply “early release”, which means that a temporarily inactive GBR bearer is released, i.e. before it is released from the application using it.
When a bearer is allocated to a user equipment, it is associated with an “Allocation and Retention Priority” value. This includes a “Pre-emption Vulnerability” bit and a “Pre-emption Capability” bit. The “Pre-emption Vulnerability” bit indicates if it is allowed to pre-empt the bearer to set up another bearer. The “Pre-emption Capability” bit indicates if it is allowed to pre-empt another bearer to set up this new bearer.
In the current 3GPP standard the base station may request the release of all bearers, i.e. all Evolved UTRAN Radio Access Bearers (E-RABs) using an S1 AP UE CONTEXT RELEASE REQUEST message sent to a Mobility Management Entity (MME). The MME is the core network node controlling the LTE access network. S1 is the name of the interface between the eNB and the EPC (Evolved Packet Core). S1 AP is the part towards the MME, comprising the control part of the S1 interface.
One reason for this request may be inactivity of an allocated bearer of a user equipment during a certain time, i.e. no data are scheduled on the bearer. This possibility is valuable when the resources in the base station, i.e. the cell resources, in terms of licensed number of connected user equipments is close to the limit. The inactivity time before a user equipment release shall be triggered is not governed by the MME and can thus be decided locally in the base station. An operator settable user equipment inactivity timer may be used for this.
In the current 3GPP standard the base station may locally release a certain radio bearer and notify the MME using the S1 AP E-RAB RELEASE INDICATION message. This option may for example be used by the base station when it detects inactivity on the bearer. The inactivity timer on user equipment level enables the release of a user equipment and all its bearers in a situation of resource shortage. As long as there is activity on some of its bearers the user equipment will remain connected. If the user equipment has unused GBR bearers, their reservation of bandwidth will remain. This prevents allocation of GBR bearers for other user equipments even if there is no GBR traffic in the cell at all. This is obviously an unsatisfactory situation even if the non used resources can be used by the non-GBR bearers.
In some markets, e.g. the US, it is not allowed to disrupt an ongoing call by pre-emption, not even for the purpose to set up an emergency call. Even if the concept of “an ongoing call” is a bit out of place in LTE, since all services are carried out in the packet domain, release of active resources must still be regarded as a forbidden action in markets not allowing pre-emption.